What was found
DIVERSION / DECLARED EMERGENCY. LEVEL AT FLT 370 EXPERIENCED A 'CABIN ALTITUDE WARNING HORN,' CABIN ALTITUDE WAS CLIMBING THROUGH 10,000' AT APPROXIMATELY 1-2000 FPM, ACCOMPLISHED CABIN ALTITUDE WARNING CHECKLIST. CABIN STILL CLIMBING INITIATED EMERGENCY DESCENT AT APPROX. 25,000'. PASSENGER 02 MASKS DID NOT DROP. REMOVED AND REPLACED OUTFLOW VALVE PER MM.